The present invention relates to a magnetic bubble memory unit, and in particular to a magnetic bubble memory unit in which the loss resistance caused by the eddy current in the conductive shield case has been reduced.
A magnetic bubble memory device is a small cylindrical magnetic domain appearing on a crystalline film such as rare-earth orthoferrite or magnetic garnet in which the internal magnetization exists perpendicular to the film surface due to strong magnetic anisotropy. The magnetic bubble memory device is controlled by the magnetic field or current fed from the outside and freely moves inside the film. Accordingly, the magnetic bubble can be used as a small-sized memory or a logic circuit.
The magnetic bubble memory device was announced by the Bell Telephone Laboratories in 1967. As compared with the magnetic tape or disk, the magnetic bubble device memory is faster in speed and higher in reliability because of absence of movable mechanical parts. As compared with the IC memory, the magnetic bubble memory is nonvolatile, higher in density, and simpler in manufacturing process. Owing to these advantages, the magnetic bubble memory is being developed as the auxiliary memory for the electronic exchange or the memory of a terminal device, a microcomputer or the like.
On the way to the invention, the inventors developed an intermediate magnetic bubble memory unit which has a structure as illustrated in FIG. 1. A chip 1 for a magnetic bubble memory is mounted on the central part of a printed circuit board 3. On the peripheral part of the board 3, a predetermined number of terminals 2 used for external connection are located. On the board 3, a magnetic core 5 made of a soft magnetic material, which takes the shape of a rectangular solid and has four windings 4 on parallel sides thereof confronting each other, is fixed so that the chip 1 will be positioned inside the magnetic core 5 and the terminals will be positioned outside the magnetic core 5. The core 5 may be omitted, though the magnetic field is weakend. The chip 1, the windings 4, the magnetic core 5, and the printed circuit board 3 excepting a part used for terminals 2 are covered by a conductive shield case 7. Only the part of the board 3 used for terminal 2 extends to the outside of the shield case 7 through an opening 6. Above and below the shield case 7, a set of a permanent magnetic plate and a magnetization alignment plate is located. The whole is covered by an annular magnetic shield case 8.
FIG. 2 is an exterior view of the above described conductive shield case 7. If only one large opening 6 for passing the external connection terminals 2 is provided as illustrated in FIG. 2, eddy currents 18 flow in the conductive shield case 7 as indicated by broken lines of FIG. 13b. Accordingly, the loss resistance R of the winding 4 illustrated in FIG. 1 is largely increased, preventing realization of a magnetic bubble memory unit having a low loss.
The rotating magnetic field for transferring the magnetic bubble in the magnetic bubble memory chip 1 is produced by letting flow orthogonal sine wave currents through two sets X and Y of windings 4. Each of the two sets X and Y is composed of two windings wound on confronting sides of the rectangular solid magnetic core 5. In this case, all the parts including the magnetic core 5 is covered by the conductive shield case 7 in order to keep an unwanted magnetic field from getting out, improve the distribution uniformity of the magnetic field, and enhance the intensity of the magnetic field. When the the eddy current 17 flows as illustrated in FIG. 13a, the conductive shield case serves to maintain the rotary field therein. However, when the eddy current flows through the portion near the opening 6 of the conductive shield case 7 as well, the eddy current distribution is not uniform and is biased as represented by the eddy current 18 of FIG. 13b. As a result, the effective loss resistance is increased. Especially, as the drive frequency f of the rotating magnetic field is increased, the effect due to the nonuniform eddy current is increased as represented by a curve b in FIG. 16 which illustrates the dependence of the loss resistance upon the frequency, resulting in a large increase in the loss resistance. In addition, when the distribution of the eddy current is not uniform in the conductive shield case, the uniformity of the magnetic field is deteriorated and the intensity of the magnetic field is lowered. The Japanese Patent Publication (Kokai) No. 16435/82 discloses a magnetic bubble module wherein a magnetic bubble chip on a circuit board is as a whole surrounded by two crossed windings.